Finish Ream AR-15 Chambers with barrel extension on?

I don't think I'll ever be in the "business" of custom chambering barrels, but as a home shop machinist and a rifle enthusiast I will probably try to do this sometime in the future. So in another thread I got great information about how people check the chamber depth while the barrel is in the lathe. As I thought more about the problem in an AR-15 barrel of matching the combination of:

barrel extension tennon

barrel extension manufacturing tolerances

bolt manufacturing tolerances

It seems to my novice brain that it would make sense to do the final chambering with the barrel extension in place and using the bolt to match to the barrel. If I were to see tolerance stacking between the chamber side of the barrel extension's locking lugs, the tennon cut for the barrel extension, and the bolt itself, then checking against a theoretical value might not get me the headspace I desire. So I would think that I would get close in the lathe, but finish either in the lathe or by hand with the barrel extension in place and using the bolt to be the final arbiter.

I know there is a lot of experience here, and so I look forward to hearing your critique of my reasoning. Chambering an AR-15 barrel may seem simple to you guys who do it every day, but it seems rather complicated to me! I'm mostly a firearms instructor and FFL and a kinda machinist so be gentle.

Thanks in advance,
-Art Kauffman (Engineering is doing something poorly, and then doing it again...less poorly)

The chamber on the barrel blank is finish reamed (slight undersized if chromed lined), and then the final headspace is set by screwing on the extension to depth and then pinned.
Even if you're just making one barrel manually on a lathe, it's still easier to adjust the extension as you can use the bolt you want to use to make the final headspace adjustment. With the extension already fixed on and reaming instead means that if you go too far, you'll have to remove the extension anyway and set if forward so fixing the extension first initially gave you no advantage.

Sillyputty, I took another look at ar15barrels.com to see the order he shows. He shows cutting the tennon and threads for the barrel extension before chambering. Do you do that, but leave it a little short at the shoulder so that you can make those final adjustments? Or do you chamber and then cut the tennon and thread?

That's correct. The extension can be advanced or backed off as needed to set the final headspace before pinning. This is easier done by leaving the shoulder a bit shy and then creep up on it.

I'm sorry, but I beg to differ with you. The barrel extension must be torqued up to the shoulder irrespective of headspace. If you torque the barrel-extension up to the shoulder and the chamber is a little too short, you need to make the chamber deeper. Backing the barrel-extension off to compensate would be a very poor choice. The pin, or pinning, you are referring to has nothing what so ever to do with holding the barrel-extension in place, it is merely to lock the barrel into a correct orientation with the upper receiver. The pin only goes into the barrel-extension, it does not lock it in place in any way.

It is easier to ream should before afixing the extension and the extension has to fit tight against the shoulder of the barrel it is NOT a barrel nut system like a savage.

The problem with Neshek's barrel is that the extension is installed and the gas port is drilled. When chambering a blank you can either 1. measure your parts to come up with a depth to ream to using a depth mike or 2. short chamber the barrel and measure the "gap" with feeler gauges when you screw on an extension till the the bolt closes.Then you ream till depth. If you go over, no drama, just readjust all pertinent dims when you shave back the chamber.

Because of the installed extension he has to know exactly how many .001's he needs to ream and get it right, it is not like a bolt gun that you can set it back a few thou's if you oops it.

If He goes to deep he has to remove the extension, set back the chamber 1 thread length (16 tpi = .0625"), remachine the thread tennon to adequate length (.619-.620"), remachine the fedramp .540"+.020", ensure that the extension "times" to TDC ( as close to exactly the same as gas port), and then try deepening it again. Screw up to many times and the gas tube will be to far back in the upper reciever (when looking at the assembly from bottom side the gas tube should be in the middle of the recess that allows the cam pin to rotate.

Thanks Skin. The barrel-extension on an AR barrel is actually the "receiver" in that it is what the bolt locks-up into. The aluminum upper receiver is there just to hold all the parts together, and to keep the bolt from falling out onto the ground!

That said, leaving the extension loose to headspace a chamber would be the exact same thing as loosening the barrel on a bolt-action rifle to headspace a chamber that is a little too shallow. Neither situation is one I would recommend.

I'm sorry if some read that pinning goes into the barrel as I have not stated that as a fact anywhere. It's just the act of putting the pin in is the last step of work on the extension end and therefore the chambering work is done.
The OP was asking if reaming was easier/better done with the extension on or off and that was what I was addressing. Reaming without the extension on is easier as it allows initial hand tightening of the extension with a bolt to show the number of degrees you're off from ideal. Then the last few thous can be removed from the shoulder for the final torque. Doing this with an extension on is much harder.

It's not the "pinning" part that concerned me, it was the part about "backing off the barrel-extension" to "headspace" the barrel, then "pinning the extension" that I was more concerned with.

The pin typically is installed when all chamber work is done, but not because the pin has any role in solidifying the location of the extension. The extension can often times even be removed with the pin still installed. The only risk you may run in doing this would be that the pin may gall the top of the threads as the extension is removed.

I don't know about all the AR manufactures, but two of them, including the #2 producer, short-chamber the barrels, install the extension and pin, then they are headspaced, by hand, with a carbide finish-reamer.

When I chamber my AR barrels I actually do the chambering first. I use a high-pressure flush system and rough the chamber first, then use a solid carbide finish reamer to complete the task. I have made a gauge that I can use in conjunction with a headspace gauge to determine the exact chamber depth. After the chamber is cut, then I form the tenon and thread it. I purchase large lots of barrel-extensions at a time, and can land the headspace right where I want it from the depth measurement taken after chambering. The extensions are consistent enough that it is VERY rare to have to make any adjustments.

Cutting the chamber a few thousandths of an inch deeper with the extension on is very quick and easy.